This invention is an improvement on the sliding sheath mechanism and the spring design for repositioning the sheath of the general type shown in U.S. Pat. No. 5,057,086 granted Oct. 15, 1991, U.S. Pat. No. 5,279,584 granted Jan. 18, 1994 and U.S. Pat. No. 5,308,332 granted May 3, 1994 to John A. B. Dillard III and James A. Orr, said patents incorporated herein in their entirety.
The prior Dillard et al. patents disclose a sliding sheath designed to automatically cover the needle of a syringe if operator loses intentional control, or when the operator finishes injection/use. A ring latch, also referred to as a locking ring mechanism, maintains the sheath in its needle-covering position so that a person cannot accidentally prick himself or another person with the newly contaminated needle. The syringe sheath is propelled to its needle-covering position by a spring, preferably helical, carried on or attached to the exterior of the syringe body. During use of the syringe the operator manually grasps the locking ring or sheath and slides the sheath rearward, which results in compression of the spring to expose the needle point. When the operator has completed injection or use of the syringe, the locking ring or sheath is manually released and the spring propels the sliding sheath forward. If it is operating properly the end of the sheath slides past the point of the needle is then locked in the protecting position. As the sheath nears the forward end of the needle the latch mechanism interacts with the syringe body to latch the sheath in its needle-covering position. Normally, the operator completely releases the sheath, and the spring force moves the sheath forward and the sheath latch mechanism activates to lock the sheath in the needle-covering position
However, a problem can arise if the operator allows the spring to gently expand too extend the sheath over the needle. The last stages of spring expansion under this condition has such diminished force that it sometimes does not actuate the latch, and the sheath can then slide rearward under the impact of a blow, exposing the needle point. To alleviate this problem a stronger spring is necessary. However, to increase the strength of the spring in these prior designs the spring requires additional turns, must be thicker in cross-section or must be constructed from a different, stronger material.
A further problem of these prior devices is that in order to provide adequate expansive power, the spring has added turns, the added turns results in a longer collapsed length and, as a result, the collapsed spring length is too great to allow the full length exposure of the needle (i.e., the length from the needle point to the needle hub) to be utilized.